The marine oils used in low-speed two-stroke crosshead engines, are of two types. On the one hand, cylinder oils, ensuring the lubrication of the cylinder-piston assembly, and, on the other hand, system oils ensuring the lubrication of all the moving parts apart from the cylinder-piston assembly. Within the cylinder-piston assembly, the combustion residues containing acid gases are in contact with the lubricating oil.
The acid gases are formed from the combustion of the fuel oils; they are in particular sulphur oxides (SO2, SO3), which are then hydrolyzed on contact with the moisture present in the combustion gases and/or in the oil. This hydrolysis generates sulphurous (HSO3) or sulphuric (H2SO4) acid. To protect the surface of piston liners and avoid excessive corrosive wear, these acids must be neutralized, which is generally done by reaction with the basic sites included in the lubricant.
An oil's neutralization capacity is measured by its BN or Base Number, characterized by its basicity. It is measured according to standard ASTM D-2896 and is expressed as an equivalent in milligrams of potash per gram of oil (also called “mg of KOH/g” or “BN point”). The BN is a standard criterion making it possible to adjust the basicity of the cylinder oils to the sulphur content of the fuel oil used, in order to be able to neutralize all of the sulphur contained in the fuel, and capable of being converted to sulphuric acid by combustion and hydrolysis.
Thus, the higher the sulphur content of a fuel oil, the higher the BN of a marine oil needs to be. This is why marine oils with a BN varying from 5 to 100 mg KOH/g are found are found on the market. This basicity is provided by detergents that are overbased by insoluble metallic salts, in particular metallic carbonates. The detergents, mainly of anionic type, are for example metallic soaps of salicylate, phenate, sulphonate, carboxylate type etc. which form micelles where the particles of insoluble metallic salts are maintained in suspension. The usual overbased detergents intrinsically have a BN in a standard fashion comprised between 150 and 700 mg KOH per gram of detergent. Their percentage by mass in the lubricant is fixed as a function of the desired BN level.
Part of the BN can also be provided by non-overbased or “neutral” detergents with a BN typically less than 150. However, the production of marine engine cylinder lubricant formulas where the entire BN is provided by “neutral” detergents cannot be envisaged: it would in fact be necessary to incorporate them in excessive quantities, which could be detrimental to other properties of the lubricant and would not be realistic from an economic point of view. The insoluble metallic salts of the overbased detergents, for example calcium carbonate, therefore contribute significantly to the BN of the usual lubricants. It can be considered that approximately at least 50%, typically 75%, of the BN of the cylinder lubricants is thus provided by these insoluble salts. The actual detergent part, or metallic soaps, found in both the neutral and overbased detergents, typically provides most of the remainder of the BN.
Environmental concerns have led, in certain areas and in particular coastal areas, to requirements relating to the limitation of the level of sulphur in the fuel oils used on ships. Thus, the regulation MARPOL Annex 6 (Regulations for the Prevention of Air Pollution from Ships) issued by the IMO (International Maritime Organization) entered into force in May 2005. It sets a global cap of 4.5% m/m on the sulphur content of heavy fuel oils as well as creating sulphur oxide emission control areas, called SECAs (Sulphur Emission Control Areas). Ships entering these areas must use fuel oils with a maximum sulphur content of 1.5% m/m or any other alternative treatment intended to limit the SOx emissions in order to comply with the specified values. The notation m/m denotes the percentage by mass of a compound relative to the total weight of fuel oil or lubricating composition in which it is included.
More recently the MEPC (Marine Environment Protection Committee) met in April 2008 and approved proposed amendments to the regulation MARPOL Annex 6. These proposals are summarized in the table below. They present a scenario in which the restrictions on the maximum sulphur content become more severe with a worldwide maximum content reduced from 4.5% m/m to 3.5% m/m as from 2012. The SECAs (Sulphur Emission Control Areas) will become ECAs (Emission Control Areas) with an additional reduction in the maximum permissible sulphur content from 1.5% m/m to 1.0% m/m as from 2010 and the addition of new limits relating to contents of NOx and particles.
Current RegulationMARPOL Annex 6General limitLimit for the SECAsMaximum sulphur content4.50% m/m1.50% m/m
Amendments to MARPOL Annex 6(MEPC Meeting No. 57 - April 2008)General limitLimit for the ECAsMaximum sulphur3.5% m/m on1% m/m oncontentJan. 01, 2012Jan. 03, 20100.5% m/m on0.1% m/m onJan. 01, 2020Jan. 01, 2015
Ships sailing trans-continental routes already use several types of heavy fuel oil depending on local environmental constraints, allowing them to optimize their operating costs. This situation will continue irrespective of the final level of the maximum permissible sulphur content of fuel oils. Thus the majority of container ships currently under construction provide for the utilization of several bunker tanks, for a “high sea” fuel oil with a high sulphur content on the one hand and for a ‘SECA’ fuel oil with a sulphur content less than or equal to 1.5% m/m on the other hand. Switching between these two categories of fuel oil can require adaptation of the engine's operating conditions, in particular the utilization of appropriate cylinder lubricants.
Currently, in the presence of fuel oil with a high sulphur content (3.5% m/m and more), marine lubricants having a BN of the order of 70 are used. In the presence of a fuel oil with a low sulphur content (1.5% m/m and less), marine lubricants having a BN of the order of 40 are used (in the future this value will be reduced). In these two cases, a sufficient neutralizing capacity is achieved as the necessary concentration in basic sites provided by the overbased detergents of the marine lubricant is reached, but it is necessary to change lubricant at each change of type of fuel oil.
Moreover, each of these lubricants has limits of use resulting from the following observations: the use of a cylinder lubricant of BN 70 in the presence of a fuel oil with a low sulphur content (1.5% m/m and less) and at a fixed lubrication level, creates a significant excess of basic sites (high BN) and a risk of destabilization of the micelles of unused overbased detergent, which contain insoluble metallic salts. This destabilization results in the formation of deposits of insoluble metallic salts (for example calcium carbonate), mainly on the piston crown, and can eventually lead to a risk of excessive wear of the liner-polishing type. Therefore, the optimization of the cylinder lubrication of a low-speed two-stroke engine then requires the selection of the lubricant with the BN adapted to the fuel oil and to the operating conditions of the engine. This optimization reduces the flexibility of operation of the engine and requires a significant degree of technical expertise on the part of the crew in defining the conditions under which the switching from one type of lubricant to the other must be carried out.
In order to simplify the operations, it would therefore be desirable to have a single cylinder lubricant for two-stroke marine engines which can be used with both high-sulphur fuel oils and low-sulphur fuel oils. In particular, a need exists for formulations in which the BN is provided, alternatively to the overbased detergents, by compounds that do not give rise to metallic deposits when they are present in excess relative to the quantity of sulphuric acid to be neutralized.
The application WO 2009/153453 discloses cylinder lubricants for two-stroke marine engines which can be used with both high-sulphur fuel oils and low-sulphur fuel oils. The lubricant compositions have a BN greater than or equal to 15, and comprise one or more lubricant base oils for marine engines, at least one overbased detergent, optionally in combination with a neutral detergent, one or more oil-soluble fatty amines. The fatty amines provide the lubricant with at least 10 BN points, and the overbased detergents at the most 20 BN points.
These lubricant compositions can have a BN of the order of 50 and are as effective from the point of view of the kinetics of neutralization of the acids as cylinder lubricants with a much higher BN (typically 70), specifically designed for high-sulphur fuels; their reduced level of overbased detergents also allows them to be adapted to low sulphur contents. However, in this type of composition, a significant part of the BN (at least 10 mg of KOH/g), is provided by the fatty amines. This high level of amine can, for certain of them, pose problems of toxicity. It results moreover in a degradation of thermal behaviour (measured in particular by their ability to form deposits in the ECBT test or “Elf Coking Bench Test” described below).
The anti-wear performances of these oils can moreover be improved. Finally, maintaining the performances of these oils throughout their residence time (approximately 30 minutes) in the cylinder poses problems.
The application FR 2094182 discloses a lubricant composition comprising from 0.01 to 5% of an acid neutralization accelerator which can be an ethoxylated fatty diamine, and sufficient alkaline-earth metal carbonate to confer a BN of 0.5 to 100 mg KOH/g on the composition. These carbonates can be dispersed in the lubricant by phenates or sulphonates.
In these compositions, virtually all of the BN is provided by alkaline-earth metal carbonates. No mention is made of the parts of the BN provided respectively by the amines, detergents, metallic carbonates. No mention is made of the presence of neutral detergents in these compositions. These compositions exhibit mediocre thermal behaviour (measured in particular by their ability to form deposits in the ECBT test).
A need therefore exists for cylinder lubricants for a two-stroke marine engine which can be used with both high-sulphur fuels and low-sulphur fuels, and the thermal behaviour and anti-wear effect of which is improved compared with the lubricants of the prior art. The present invention relates to a lubricant composition which can be used as cylinder lubricant for two-stroke marine engines, which can be used with both high- and low-sulphur fuels, and which makes it possible to remedy the abovementioned drawbacks. The lubricant compositions according to the invention comprise alkoxylated fatty amines in limited quantities, combined with neutral detergents and overbased detergents in specific proportions. They are as effective from the point of view of acid neutralization kinetics as cylinder lubricants with a much higher BN (typically 70), specifically designed for high-sulphur fuels; their reduced level of overbased detergents also allows them to be adapted to low-sulphur fuels.
The compositions according to the invention exhibit very good anti-wear properties, and better thermal behaviour than the compositions of the prior art. They resist ageing better and retain these properties throughout their residence time in the cylinder of the marine engine.